Waveguide-type polarization converter, a device which can rotate polarized light to a specific direction, has been widely applied in the field of integrated optics. In this regard, many attempts to develop a more efficient polarization converter have been made, in a view of recent development of integrated optics.
For example, TE/TM(transverse electric/transverse magnetic) polarization mode converters have been fabricated by LiNbO.sub.3 and InP waveguides(see: R. C. Alferness et al., Appl. Phys. Lett., 38(9):655-657(1981); M. Schlak et al., Electron. Lett., 22(17):883-885(1986)).
F. Heismann et al suggest a polarization-independent photonic switching system which is operated by controlling polarized light with the said TE/TM polarization mode converter, which can operate only when voltage is applied from outside(see: F. Heismann et al., IEEE Photon. Technol. Lett., 5(11):1341-1343(1993)).
In this regard, studies on the development of a passive polarization converter which can operate without voltage application have been actively carried out, and the following polarization converters have been proposed in the art:
CA 2,068,439 discloses a passive polarization converter in which a periodic coupling between two guided nodes of an optical signal propagating in the waveguide takes place, which can transform signal propagation mode.
Y. Shani et al teach a passive polarization converter fabricated by employing a periodically tilted rib waveguide and an asymmetric periodic load(see: Y. Shani et al., Appl. Phys. Lett., 59(11):1278-1280(1991)).
H. Heidrich et al describe a passive polarization converter equipped with a InP/GaInAsp waveguide which is periodically tilted(see: H. Heidrich et al., IEEE Photon. Technol. Lett., 4(1):34-36(1992)).
Aforementioned passive polarization converters of prior art are known to be devices which are fabricated to change cladding or core structure of a waveguide periodically, finally to bring about polarization coupling between polarized light. Said prior art polarization converters, however, have been proven less satisfactory in the sense that precise control is essentially required for the polarization conversion and scattering by the perturbation of periodic structures results in propagation loss of a device.